Two of my most favorite lens, back when I was still using Canon's DSLR are Canon's own 100mm F2.8 L lens, and the zeiss 50mm F2. The Canon 100mm F2.8 L Macro was the first lens that were stellar enough, it made me a convert to prime lenses. I used to own several zoom lenses and just 1 macro lens, which was Tamron 90mm, after getting the canon, I ditch all the zoom lens and the Tamron's macro.

Not long after, I got the more exotic Zeiss 50mm Makro, which was a full manual lens. This lens made my 100mm enter an early retirement, from being present most of time in my back pack, to just being used on special occasion a few times a year.

This photos were taken RAW and without software correction. Hence the result would be a tad under, since in RAW files there's simply no enhancement of contrast, saturation or sharpness, such as the result of shooting in JPEG mode.

The most recent lens in my collection, is the venerable Sony 90mm. It was touted as the sharpest lens ever by DXOMark (at least of every lens tested on DXOMark). I decided to purchase it, though it costs an arm and a leg to buy. At about $1100 it certainly isn't cheap, but I figure it's still worth it to find out what's the sharpest lens look like.

Shots were done manually, focusing on the eye. The ambient light were quite dim and the autofocus on Sony A7 weren't stellar at low light. So I ended up doing it the old fashion way. This two shots were the sharpest ones out of the bunch taken.

A7 body was mounted on a sturdy Manfrotto 190 XproB tripod

For the low light result, I much preferred the Sony's image. It's brighter while still pleasing for portrait. Keep in mind that the Canon were used with adapter, so this supposedly would contribute to the loss of brightness in the final image.

This burger shots are taken early in the morning where ambient light were plenty, and using tripod.
The distance were the same, so there's a 10mm reach difference between the two lenses.
You can draw your own conclusion from this shots above. I myself feel that there's an advantage in sharpness, area in focus and micro contrast. The focus point were taken at the burger patty closest to the camera /lens, just above the onion. Having said that, after post processing, the final result would probably not vary much.

I'm a sucker for sharp lens, so if I don't have the Canon 100mm macro, I'd choose the 90mm even though the price is much steeper.

For an ex Canon user that decided to move to Sony A7, I'd recommend sticking to Canon 100mm and using adapter rather than buying Sony 90mm.The sony has much pleasing result for low light, but I found myself rarely using the macro lenses for low light condition.
It's a different case if you're buying the Sony A7ii, A7Sii and A7Rii which have much faster AF, the price difference would justify the increased performance.